What is burn-through in MIG welding?

• A. Melting completely through the base metal
• B. Surface oxidation only
• C. A hairline crack
• D. A misalignment

Answer: (A)

Burn-through happens when the heat from the MIG welding arc is enough to melt all the way through the base metal, creating a through-hole rather than just filling a joint. In practice, this means the weld pool penetrates the full thickness and breaks through to the other side or to backside of the workpiece. It’s more likely on very thin materials, with too high current, too long an arc length, or slower travel speed, and it often leaves a visible hole or thinning where the metal has been consumed. This isn’t just surface discoloration from oxidation, nor a hairline crack, nor a misalignment. It’s actual full-thickness penetration and loss of material. To prevent it, you’d reduce heat input by lowering current or shortening the arc length, increase travel speed, adjust joint design, and ensure appropriate backing or support for the weld.